1. Field of the Invention
The present invention relates to a light emitting device and an information processing apparatus for optical recording/reproduction of information.
2. Description of the Related Art
When a single apparatus is used to record/reproduce information for a plurality of recording mediums which have different recording density of information, in general, an information processing apparatus having a plurality of light sources is used. For example, for recording mediums such as a Digital Versatile Disc (hereinafter referred to as “DVD”) and a compact disk (hereinafter referred to as “CD”) which have different formats, a technique for recording/reproduction of information using a single apparatus having two light sources with different wavelengths has been developed and is practically used.
A light emitting device is used in many practical apparatuses having a plurality of light sources. Conventionally, such a light emitting device has a housing which accommodates a single light source. Accordingly, a plurality of light emitting devices are required for a plurality of light sources.
Recently, Japanese Laid-open publication No. 2002-190133 proposes an apparatus including a light emitting device having a single housing which accommodates two light sources for emitting light having different wavelengths (hereinafter referred to as “two wavelength light source device”). According to the light emitting device, the structure of the apparatus can be simplified, the cost of the apparatus can be reduced due to the reduction of the adjustment steps of parts and assemblies, or high reliability of the apparatus can be achieved.
FIG. 5 shows an exemplary structure of an apparatus including a two wavelength light source device. In FIG. 5, 51a denotes a first light source for emitting light having a wavelength λ1 and 51b denotes a second light source for emitting light having a wavelength λ2. The light sources 51a and 52b are arranged within a two wavelength light source device 51.
The light 2 emitted from the light source 51a is reflected by a beam splitter 4, and is converted into substantially collimated light by a collimating lens 5. The substantially collimated light is converged to a first recording medium 7 by an objective lens 6.
The reflected light from the first recording medium 7 reaches the beam splitter 4 through the opposite path. A portion of the light is transmitted through the beam splitter 4 and enters into a light detector 10 via a detection lens 9. Various signals such as a focus signal, a tracking signal and an RF signal can be detected based on the light entering into the light detector 10.
Herein, the structure of the light detector 10 and/or a detection system for various signals are not essential to the invention, since they are already known in public. Therefore, the detailed description thereof will be omitted.
The light 3 emitted from the second light source 51b also reaches the objective lens 6 through the same path as the light 2 emitted from the first light source 51a and converged to a second recording medium 8. Because the first recording medium 7 and the second recording medium 8 have different thickness of the substrates, the objective lens 6 has a structure in which any spherical aberration is corrected for each thickness of the substrates.
Herein, the structure of the objective lens 6 and/or the method for correcting the spherical aberration are not essential to the invention, since they are already known in public. Therefore, the detailed description thereof will be omitted.
The reflected light from the second recording medium 8 reaches the beam splitter 4 through the opposite path. A portion of the light is transmitted through the beam splitter 4 and enters into a light detector 10 via a detection lens 9. Various signals such as a focus signal, a tracking signal and an RF signal can be detected based on the light entering into the light detector 10.
Again, the structure of the light detector 10 and/or a detection system for various signals are not essential to the invention, since they are already known in public. Therefore, the detailed description thereof will be omitted.
Recently, needs for recording information on the recording medium such as CD and DVD at high speed have been increased. In order to accommodate those needs, it is necessary to output light to the objective lens, wherein the light has a high power sufficient to perform high speed recording.
The angle (full-width-half-maximum) of radiation of light emitted from a semiconductor laser having a high power output, which is currently used as a light source for CD recording or a light source for DVD recording, is between 7 to 10 degrees in the horizontal direction and between 16 to 23 degrees in the vertical direction. Thus, the light source for CD recording and the light source for DVD recording have substantially the same radiation characteristics.
It is assumed that an angle of radiation of light emitted from a light source and a Numerical Aperture (hereinafter referred to as “NA”) of an objective lens are the same. In this case, as an optical magnification fc/fo is smaller, the efficiency of light becomes higher. Herein, fc denotes a focusing distance of the collimating lens and fo denotes an focusing distance of the objective lens. This enables to efficiently introduce the light from the semiconductor laser into the objective lens.
For this reason, a smaller fc/fo is more advantageous to realize an apparatus corresponding to high-speed recording, which requires that the objective lens outputs light having a high power.
However, making the fc/fo much smaller reduces the rim strength causing some adverse effects such as making the diameter of the focusing spot become larger, dropping signal quality of the reflected light from the recording medium and not obtaining sufficient apparatus performance.
For this reason, fc/fo=4-5 is adopted as a range of the optimal optical magnification when a common semiconductor laser for CD recording is used.
In DVD recording/reproducing, the recording density of information per unit area is higher than for a CD. Therefore, the focusing spot is needed to be narrower than for a CD. For that reason, the wavelength of the semiconductor light source is made shorter than the light source for CD (CD: substantially 0.78 μm, DVD: substantially 0.60-0.67 μm) and the NA of the objective lens is made larger (CD: substantially 0.45-0.53, DVD: substantially 0.60-0.67). Also the rim strength needs to be higher than the optical system for CD recording. Therefore, fc/fo for DVD is higher than that for CD and fc/fo=6-7 is optimum when currently a common semiconductor laser for DVD recording is used.
However, because an optical system from a light source 10a to the objective lens 6 and an optical system from a light source 10b to the objective lens 6 are identical, an optical magnification fc/fo is substantially identical (to be precise, because of differences of refractive index for each wavelength, and the focusing distance of the lens for each wavelength are slightly different), thus, each cannot obtain the optimum fc/fo. Accordingly, when the optimum fc/fo is different, such as the optical system for DVD recording and the optical system for CD recording, there is a problem that a CD high-speed recording performance and recording/reproducing performance resulting from a focusing performance of a focusing spot for DVD are not compatible.
Specifically, when a plurality of light sources use a light emitting device arranged within a same housing and optical systems having different optimum optical magnifications (fc/fo), such as the ones used for an optical system for DVD recording and an optical system for CD recording, are composed of identical optical systems, the conventional technique can not obtain an optimum fc/fo for each optical system. Therefore, when an optical magnification is made to a value close to the optimum fc/fo for an optical system for DVD recording, the efficiency from a light source of a CD becomes lower and a high-speed recording performance can not be obtained. On the other hand, when an optical magnification is made to a value close to the optimum fc/fo for an optical system for CD recording, the rim strength of light incident on an objective lens from a light source for DVD drops, a focusing performance of the focusing spot of DVD drops and therefore, the desired DVD record-reproducing performance is not able to be obtained.
The present invention solves the conventional problems. Using a plurality of light sources of different wavelengths, it is intended to improve a recording-reproducing performance for a recording medium with a plurality of kinds of different specifications.